Effects of floral sexual investment and dichogamy on floral longevity

Aims Floral longevity, the duration that a flower remains open and functional, varies greatly among species. Variation in floral longevity has been considered to be optimal strategy for resource allocation under different ecological conditions, mainly determined by the rates of pollination and cost of flower maintenance. However, it is unclear whether an intrinsic factor, floral sexual investment, constrains evolution of floral longevity. The theoretical model also predicts that dichogamy favors long-lived flowers, but empirical studies to test this prediction remain unexplored.Methods To examine the effect of floral sexual investment on floral longevity, we measured flower size together with pollen and ovule production in 37 sympatric flowering plants in a natural community. The duration of the female and male phase in 21 protandrous species and floral longevity of the other 16 adichogamous species were documented in the field.Important findings Floral longevity varied from 1 day to 15 days, while pollen number per flower varied from 643 to 710880 and ovule number per flower from 1 to 426 in the 37 species. Flower size was correlated with pollen production as well as ovule production. Floral longevity was positively related to pollen production but not to ovule production. Consistent with the prediction that dichogamy favors long-lived flowers, we found the floral longevity of protandrous species was significantly longer than that of adichogamous species. In the protandrous species, pollen production per flower was observed to be positively related to male duration, while ovule production was not related to female duration. Our analyses of variation in floral longevity and sexual investment among different species suggest that the floral sexual investment could be an intrinsic factor contributing to the selected floral longevity, particularly the male phase, and that high pollen production could potentially increase pollen removal, i.e. male productive success.     

Mutation and sexual selection: a test using barn swallows from Chernobyl

Abstract Secondary sexual characters have been hypothesized to be particularly susceptible to the deleterious effects of mutation because the expression of such characters is usually influenced by many more metabolic pathways than are ordinary morphological characters. We tested this hypothesis using the elevated mutation rates in the barn swallow ( Hirundo rustica ) of the Chernobyl region of Ukraine as a model system. A great deal is known about the relative importance of different characters for male mating success in this species. The importance of phenotypic characters for male mating success was quantified based on a long-term study of a Danish breeding population, by expressing phenotypic differences between mated and unmated males as the difference between log-transformed mean values. For field samples from Ukraine we likewise expressed the difference in male phenotype between individuals living in a relatively uncontaminated area and individuals from the Chernobyl region as the difference between log-transformed mean values. The standardized difference in male phenotype between the two regions in Ukraine for the 41 different characters was strongly positively correlated with the standardized difference in male phenotype between mated and unmated males from Denmark. The standardized difference in male phenotype between the two regions in Ukraine was significantly positively associated with sexual size dimorphism. However, the standardized difference in male phenotype between mated and unmated males was a much better predictor of standardized difference in male phenotype between the two regions in Ukraine than was the standardized difference in sexual size dimorphism, expressed as the difference between log-transformed mean values for males and females. These findings are consistent with the hypothesis that traits most important for sexual selection are particularly susceptible to the effects of deleterious mutations.     

Sexual size dimorphism and sexual selection in turtles (order testudines)

Summary This paper combines published and original data on sexual size dimorphism, reproductive behavior, and habitat types in turtles. Our major finding is that observed patterns of sexual size dimorphism correlate with habitat type and male mating strategy. (1) In most terrestrial species, males engage in combat with each other. Males typically grow larger than females. (2) In semiaquatic and bottom-walking aquatic species, male combat is less common, but males often forcibly inseminate females. As in terrestrial species, males are usually larger than females. (3) In truly aquatic species, male combat and forcible insemination are rare. Instead, males utilize elaborate precoital displays, and female choice is highly important. Males are usually smaller than females.We interpret these correlations between sexual behavior and size dimorphism in terms of sexual selection theory: males are larger than females when large male size evolves as an adaptation to increase success in male combat, or to enable forcible insemination of females. In contrast, males are usually smaller than females where small size in males evolves to increase mobility (and hence, ability to locate females), or because selection for increased fecundity may result in increased female size. In turtle species with male combat or forcible insemination, the degree of male size superiority increases with mean species body size.     

Alternative mating tactics and extreme male dimorphism in fig wasps

The dimorphisms in morphology and behaviour of male fig wasps are among the most extreme in the animal kingdom, and offer excellent opportunities to test the predictions of certain sexual selection models. Winged males resemble their conspecific females closely, but wingless males are so divergent in form that they have repeatedly been classified into different taxa. Wingless males mate within their natal fig fruits, whereas winged males disperse to mate. Individual species may have winged males, wingless males or both morphs. A key hypothesis proposes that sexual selection on male mating opportunities favours winged males in species with small broods and wingless males in species with large broods. Using data from 114 species in 33 genera, we show that both simple and formal comparative analyses support the correlated evolution of large brood size and male winglessness. Theoretical models further predict that, in male dimorphic species, the proportion of winged males should equal (in cases without local mate competition) or exceed (in cases with local mate competition) the proportion of females developing in fig fruits without wingless males. These predictions are met by eight out of nine male dimorphic species studied. Taken together, the patterns across all species, and between different male dimorphic species, strongly support sexual selection on mating opportunities as the major determinant of male morph ratios in fig wasps.     

Conspicuous Coloration in Males of the Damselfly Nehalennia irene (Zygoptera: Coenagrionidae): Do Males Signal Their Unprofitability to Other Males?

In damselflies, sexual colour dimorphism is commonly explained as a consequence of selection on traits that increase male attractiveness to females. However, while many species in the damselfly family Coenagrionidae (Insecta: Odonata) are sexually dimorphic, the males do not engage in displays, and male competition for mates resembles a “scramble”. An alternative explanation for the sexual differences in coloration within these species is that sexual dimorphism has evolved as a sex-related warning signal, with males signalling their uprofitability as mates to other males, thereby avoiding harassment from conspecifics. We evaluated an underlying assumption of the theory that male-male harassment rate is influenced by colour by comparing harassment of males of the species Nehalennia irene that had been painted to make them appear: (i) similar to an unaltered male (blue), (ii) different from a male (orange) and (iii) more similar to a female (black). When caged together we found that blue-painted males experienced significantly lower harassment than black-painted males. When unpainted males were caged with each type of painted male we found that blue-painted males and the unpainted males housed in the same cages experienced lower rates of harassment than males housed in cages where some males were painted black, suggesting that a single, reliable signal of unprofitability may benefit the individuals that carry it. While our results do not in themselves demonstrate that sexual colour dimorphism originally evolved as an intra-specific warning signal, they do show that harassment is influenced by coloration, and that such selection could conceivably maintain male coloration as a warning signal.     

Constraints on the evolution of males and sexual dimorphism: field estimates of genetic architecture of reproductive traits in three populations of gynodioecious Fragaria virginiana

Abstract To understand how genetic constraints may limit the evolution of males and sexual dimorphism in a gynodioecious species, I conducted a quantitative genetic experiment in a gynodioecious wild strawberry, Fragaria virginiana . I estimated and compared genetic parameters (narrow-sense heritabilities, between-trait and between-sex genetic correlations, as well as phenotypic and genetic variance-covariance matrices) in the two sex morphs from three populations grown in a common field garden. I measured pollen and ovule production per flower, petal size, fruit set, and flower number. My major findings are as follows. (1) The presence of a phenotypic trade-off between pollen production and fruit set in hermaphrodites reflects a negative genetic correlation in the narrow sense that is statistically significant when pooled across populations. (2) The main constraints on the evolution of males are low genetic variation for pollen per flower and strong positive correlations associated with ovule number (e.g., between pollen and ovules in hermaphrodites, and between ovules in hermaphrodites and females). (3) Traits with the lowest levels of sexual dimorphism (ovule number and flower number) have the highest between-sex genetic correlations suggesting that overlap in the expression of genes in the sex morphs constrains their independent evolution. (4) There are significant differences in G matrices between sex morphs but not among populations. However, evidence that male-female trait correlations in hermaphrodites were lower in populations with higher frequencies of females may indicate subtle changes in genetic architecture.     

CONSUMER DRIVEN POLLEN LIMITATION OF SEED PRODUCTION IN MARSH GRASSES

While flower predators can limit the sexual expression and seed production of salt marsh grasses, the relationship between these two effects of consumers has not been explored. At our study site, predation on Spartina patens, Spartina alterniflora, and Distichlis spicata was twice as high in 1985 (~70% ovule destruction) as in 1986 (~35% ovule destruction). In both years consumers destroyed flowers before maturity, reducing sexual expression, and particularly suppressed male sexual expression. Sexual suppression of males was much more pronounced in 1985 when flower predation was severe and the seed production of undamaged ovules was dramatically reduced. A number of lines of evidence suggest that predator limitation of male sexual expression and pollen supply contributed to low seed output in 1985. 1) Undamaged ovules of all three grasses protected from consumers but exposed to ambient windbome pollen set many more seeds in 1986 than in 1985, suggesting that pollen was more abundant in 1986; 2) Artificial pollinations revealed that marsh grasses are generally pollen-limited and that pollen limitation at our study site was more severe in 1985 than 1986; and 3) Caging stands of marsh grasses generally led to less predator damage, increased male densities and seed sets similar to those for hand-pollinated flowers. Our results support the hypothesis that flower predators can indirectly limit seed production by decreasing pollen availability.     

The evolution of male infanticide in relation to sexual selection in mammalian carnivores

The evolution of infanticide by males has often been explained by the sexual selection hypothesis, which posits that infanticide improves male reproductive success by shortening the interbirth intervals of the mothers of the killed offspring. In Carnivora, however, the fitness advantages assumed in this hypothesis have been shown in only a few species, and it has been argued that male infanticide may be nonadaptive in pinniped carnivores. According to the sexual selection hypothesis, male infanticide is expected to be more prevalent in species in which males are subjected to stronger sexual selection through intrasexual competition over mates. We examined a phylogenetically corrected relationship between male infanticide and sexual size dimorphism (SSD) as a measure of the intensity of sexual selection in carnivores. Our analyses failed to detect a significant association between the occurrence of male infanticide and SSD across carnivores, although they showed that, among fissipeds (typically terrestrial carnivores), males in species with stronger male-biased SSD are significantly more likely to commit infanticide. This suggests that the evolution of male infanticide is correlated with intense sexual selection in fissipeds. In pinnipeds (Odobenidae, Otariidae, and Phocidae), there was no significant association between male infanticide and SSD. Assuming that SSD represents the intensity of sexual selection on males, this result is consistent with the argument that infanticide by male pinnipeds is not a sexually selected behaviour.     

Dimorphic male midshipman fish: reduced sexual selection or sexual selection for reduced characters?

In most taxa with male dimorphisms, some males are large inbody size with exaggerated secondary sexual characters (exaggeratedmorph), whereas other males in the same population are smalland have reduced secondary sexual characters (reduced morph).What selective pressures cause male dimorphisms? Reduced morphologiesmay result when a) some males develop a morphology that, inthe absence of sexual selection pressures for an exaggeratedmorphology, reduces energetic and developmental costs and/orb) some males opt for an alternative morphology that does wellat an alternative behavioral tactic such as cuckoldry. The 2mechanisms could act together, but each alone is theoreticallysufficient to drive dimorphisms. Here, we tested hypothesis"b" (sexual selection for reduced characters) in the plainfinmidshipman fish, Porichthys notatus. Behavioral plasticity betweenterritoriality and cuckoldry in an exaggerated male morph (typeI) allows for a direct comparison of cuckoldry by exaggeratedmorph males to cuckoldry by reduced morph (type II) males. Comparedwith type I cuckolders, type II cuckolders were able to remainnear the nest for longer periods before being chased by theterritorial type I male, suggesting that the reduced type IImorphology allows type II males to prolong the time before attackby territorial males. Combined with other studies showing arole of sexual selection in maintaining the exaggerated morph,the data support the "sexual selection for reduced characters"hypothesis and elucidate how sexual selection can act in differentways on different males to maintain 2 male morphologies withina single species.     

Heterochrony and its role in sex determination of cryptically dioecious Consolea (Cactaceae) staminate flowers

Ovule development, megasporogenesis, and megagametogenesis were studied in six cryptically dioecious species of Consolea. All species showed uniform development typical for the Opuntioideae. Ovule development proceeds acropetally, but shows developmental asynchrony across floral morphs. At anthesis, female morph ovules are functional and available for fertilization, whereas staminate flower ovules are senescing and incapable of being fertilized. In occasional plants of some species, staminate flowers may reach anthesis with a few functional apical ovules capable of seed formation. Such plants are described as inconstant/leaky males. Ovule fertility differences across morphs are interpreted as resulting from heterochronic ovule development and senescence, although variation in embryo sac longevity cannot be ruled out. Significantly, ovule abortion follows a common pattern and timing in staminate flowers of both male morphs in all species. Thus, on the basis of this uniformity, a common origin for the cryptically dioecious breeding system in Consolea is hypothesized. Furthermore, staminate expression in Consolea appears to be controlled by a common, genetically determined heterochronic ovule developmental programme affecting the relative timing of ovule receptivity and flower opening. This is the first report of heterochrony as a mechanism of male sex determination.  © 2008 The Linnean Society of London, Botanical Journal of the Linnean Society , 2008, 156 , 305–326.     

Evolution of mating system and the genetic covariance between male and female investment in Clarkia (onagraceae): selfing opposes the evolution of trade-offs

Sex allocation theory has assumed that hermaphroditic species exhibit strong genetically based trade-offs between investment in male and female function. The potential effects of mating system on the evolution of this genetic covariance, however, have not been explored. We have challenged the assumption of a ubiquitous trade-off between male and female investment by arguing that in highly self-fertilizing species, stabilizing natural selection should favor highly efficient ratios of male to female gametes. In flowering plants, the result of such selection would be similar pollen:ovule (P:O) ratios across selfing genotypes, precluding a negative genetic correlation (r(g)) between pollen and ovule production per flower. Moreover, if selfing genotypes with similar P:O ratios differ in total gametic investment per flower, a positive r(g) between pollen and ovule production would be observed. In outcrossers, by contrast, male- and female-biased flowers and genotypes may have equal fitness and coexist at evolutionary equilibrium. In the absence of strong stabilizing selection on the P:O ratio, selection on this trait will be relaxed, resulting in independence or resource-based trade-offs between male and female investment. To test this prediction, we conducted artificial selection on pollen and ovule production per flower in two sister species with contrasting mating systems. The predominantly self-fertilizing species (Clarkia exilis) consistently exhibited a significant positive r(g) between pollen and ovule production while the outcrossing species (C. unguiculata) exhibited either a trade-off or independence between these traits. Clarkia exilis also exhibited much more highly canalized gender expression than C. unguiculata. Selection on pollen and ovule production resulted in little correlated change in the P:O ratio in the selfing exilis, while dramatic changes in the P:O ratio were observed in unguiculata. To test the common prediction that floral attractiveness should be positively genetically correlated with investment in male function, we examined the response of petal area to selection on pollen and ovule production and found that petal area was not consistently genetically correlated with gender expression in either species. Our results suggest that the joint evolutionary trajectory of primary sexual traits in hermaphroditic species will be affected by their mating systems; this should be taken into account in future theoretical and comparative empirical investigations.     

Testing the role of male–male competition in the evolution of sexual dimorphism: a comparison between two species of porcelain crabs

Theory predicts marked sexual dimorphism in terms of body size and body structures used as weapons (e.g. chelipeds) in gonochoric species with intense male sexual competition for receptive females and reduced or no sexual dimorphism in species where competition among males is trivial. We tested this hypothesis using a pair of closely‐related species of symbiotic porcelain crabs as a model. In one species that inhabits sea anemones solitarily, competition among males for receptive females is unimportant. In a second species that dwells as dense aggregations on sea urchins, male–male competition for sexual partners is recurrent. We expected considerable sexual dimorphism in body size and weaponry in the urchin‐dwelling crab and reduced sexual dimorphism in the anemone‐dwelling crab. In agreement with expectations, in the urchin‐dwelling crab, male body size was, on average, larger than that of females and males invested considerably more to cheliped length than females. Also supporting theoretical considerations, in the anemone‐dwelling crab, sexual dimorphism in terms of body size was not detected and differences between the sexes in investment to cheliped length were minor. Interestingly, chelipeds were more developed both in males and females of the anemone‐dwelling crab than in the urchin‐dwelling crab as a result of the importance of these structures for monopolization of their naturally scarce anemone hosts. Another difference between the studied species was the existence of two clearly distinguishable ontogenetic phases in males of the urchin‐dwelling crab but not in males of the anemone‐dwelling crab. Whether the two different male morphs display different male reproductive strategies in the urchin‐dwelling crab remains to be addressed. Other conditions that might additionally explain the observed differences in sexual dimorphism (e.g. female mate choice) between the studied species remain to be explored. © 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, 105 , 548–558.     

Plumage redness predicts breeding onset and reproductive success in the House Finch: a validation of Darwin's theory

Darwin (1871) and later Fisher (1958) suggested that sexual selection can drive the evolution of ornamental traits in monogamous species when female preferences for these traits allow well-ornamented males to begin breeding earlier in a season and, as a result, gain reproductive advantages over poorly ornamented males. However, few studies have been conducted to test this fundamental concept upon which much of the sexual selection theory for monogamous species has been based. In this study, we examined the relationship between breeding onset, reproductive success, and male ornamentation in the House Finch Carpodacus mexicanus , a species in which males display bright carotenoid-based plumage pigmentation. In previous work, it has been shown that bright male House Finches are preferred as social mates by females and, as a result, begin nesting earlier in the season than do drab orange and yellow males. Here we show that, by initiating breeding earlier in the season, brightly colored males fledge more offspring in a season than do drab males. Thus, differential timing of breeding generates considerable variance in reproductive success among male House Finches and contributes to sexual selection for male plumage ornamentation in this species.     

Hormonal induction of undescribed males resolves cryptic species of cladocerans

Cyclic parthenogens have a mixed breeding system with both meiotic and ameiotic eggs. Although investment in sexual stages is often synchronized with seasonal cycles, the degree of investment is a quantitative trait associated with habitat instability. Populations of cyclic parthenogens from stable environments, such as large lakes and oceans, generally show reduced or undetectable investment in males. Indeed, males of many species of lacustrine cyclic parthenogens are unknown to science. Methyl farnesoate (MF), a crustacean juvenile hormone, has been implicated as an inducer of male formation in Daphnia magna (Crustacea: Cladocera), a denizen of unstable habitats with marked sexual recruitment. Here, we show experimentally that MF induces male production in four distantly related lacustrine species of cladocerans under growth conditions unfavourable for male production. The males of three species are new to science. Unlike females, the anatomy of the previously unknown males of Bosmina (Lunobosmina) oriens permitted ready morphological diagnosis of sibling species and subfossils. The results suggest that the role for MF in the sex determination of cladocerans is general.     

Sexually selected dichromatism in the hihi Notiomystis cincta: multiple colours for multiple receivers

Why do some bird species show dramatic sexual dichromatism in their plumage? Sexual selection is the most common answer to this question. However, other competing explanations mean it is unwise to assume that all sexual dichromatism has evolved by this mechanism. Even if sexual selection is involved, further work is necessary to determine whether dichromatism results from competition amongst rival males, or by female choice for attractive traits, or both. Here, we test whether sexually dichromatic hihi (Notiomystis cincta) plumage is currently under sexual selection, with detailed behavioural and genetic analyses of a free‐living island population. Bateman gradients measured for males and females reveal the potential for sexual selection, whilst selection gradients, relating reproductive success to specific colourful traits, show that there is stabilizing selection on white ear tuft length in males. By correlating colourful male plumage with different components of reproductive success, we show that properties of yellow plumage are most likely a product of male–male competition, whilst properties of the black and white plumage are an outcome of both male–male competition and female choice. Male plumage therefore potentially signals to multiple receivers (rival males and potential mates), and this may explain the multicoloured appearance of one of the most strikingly dichromatic species in New Zealand.     

Genetics of sex allocation in Mimulus (Scrophulariaceae)

Theoretical models of the evolution of resource allocation patterns to male and female function make the assumption that there are inherent trade-offs between the two. Here we use a quantitative genetic approach to quantify trade-offs between male and female function and to determine whether plant populations could readily respond to natural selection by quantifying the amount of genetic variation for pollen and ovule production. Both intra- and interspecific crossing designs were applied to two populations of the predominantly outcrossing Mimulus guttatus and two populations of the highly selfing congener, M. micranthus. The only significant correlations observed among pollen number, pollen size and ovule number were positive. Positive genetic correlations among the traits were sometimes reduced after removing the effect of flower size but still no significant negative correlations were detected. These results suggest that positive correlations between pollen and ovule production may be due to the joint positive correlation of these characters with the resource pool available for pollen and ovule production, as reflected by flower size. Heritabilities were moderate to high for ovule production but low for pollen number and pollen size and suggest that responses to selection would differ between the two traits. Crosses between the species revealed that there are additional genetic factors contributing to differences between the two species for corolla width, vs. pollen:ovule ratio. This is consistent with the hypothesis that genetic variation for resource acquisition may in part be responsible for the overall lack of a negative correlation between pollen and ovule production and provides a genetic explanation for little evidence of trade-offs between sexual functions in Mimulus.     

Population density and structure drive differential investment in pre‐ and postmating sexual traits in frogs

Sexual selection theory predicts a trade‐off between premating (ornaments and armaments) and postmating (testes and ejaculates) sexual traits, assuming that growing and maintaining these traits is costly and that total reproductive investments are limited. The number of males in competition, the reproductive gains from investing in premating sexual traits, and the level of sperm competition are all predicted to influence how males allocate their finite resources to these traits. Yet, empirical examination of these predictions is currently scarce. Here, we studied relative expenditure on pre‐ and postmating sexual traits among frog species varying in their population density, operational sex ratio, and the number of competing males for each clutch of eggs. We found that the intensifying struggle to monopolize fertilizations as more and more males clasp the same female to fertilize her eggs shifts male reproductive investment toward sperm production and away from male weaponry. This shift, which is mediated by population density and the associated level of male–male competition, likely also explains the trade‐off between pre‐ and postmating sexual traits in our much broader sample of anuran species. Our results highlight the power of such a multilevel approach in resolving the evolution of traits and allocation trade‐offs.     

Canine tooth size variability in primates

I present an analysis of canine tooth size variability in male and female primates. The coefficient of variation (CV = SD X 100/mean) as an index of canine size variability proved to be dependent on mean canine size in males and, to a lower extent, in females. Therefore, variability tends to increase with increasing values of mean canine size. Using residuals from the regression of log SD on log mean canine size in male and female primates, I analysed the contribution of diet, habitat and mating system to canine size variability. Habitat and mating system are known to influence to a certain extent the degree of sexual dimorphism in canine size. Given the well-known relationship between sexual dimorphism and phenotypic variability, it was suggested that these factors might influence variability in canine size. Everything else being equal, males of polygynous species are characterized by more variable canine sizes than males of monogamous species. Habitat and diet did not contribute to the level of variability observed in either males or females. It is proposed that a high level of variability in canine size may be related to the likelihood that enlarged canines evolved as a result of male-male competition for mates in polygynous species.     

The Role of Body Size in Mating Interactions of the Sexually Cannibalistic Fishing Spider Dolomedes triton

Some arachnids display extreme sexual size dimorphism (SSD) with adult females being several times larger than adult males. One explanation for SSD in species that exhibit pre‐copulatory sexual cannibalism (female attack, kill and consumption of the male prior to mating) is that smaller males may be less likely victims of predatory attacks by females. However, in some sexually cannibalistic species SSD is relatively moderate (i.e. males are similar in size to females) suggesting benefits of large male body size. Here, I report the results of an experiment designed to explore the ramifications of body size in mating interactions of the sexually cannibalistic, North American fishing spider (Dolomedes triton). Results suggest that male size does not influence courtship behavior, the likelihood of being attacked, or the male's ability to secure a mounting. However, large males were superior at gaining copulations once mounted. Sexual cannibalism may also be predicated on female size. Female condition (mass/cephalothorax area) did not explain any of these behaviors from the copulatory sequence, however, females with a smaller cephalothorax area were more likely to attack courting males. Finally, analysis of the ratio of female size to male size showed that when SSD is weak males are more likely to escape attacks and mate successfully. Results are discussed in light of several hypotheses for sexual cannibalism, and the benefits of large male body size illustrated here are put forth as potential explanations for the relatively moderate extent of SSD found in this sexually cannibalistic species.     

Preference for conspecifics evolves earlier in males than females in a sexually dimorphic radiation of fishes

Speciation by sexual selection is generally modeled as the coevolution of female preferences and elaborate male ornaments leading to behavioral (sexual) reproductive isolation. One prediction of these models is that female preference for conspecific males should evolve earlier than male preference for conspecific females in sexually dimorphic species with male ornaments. We tested that prediction in darters, a diverse group of freshwater fishes with sexually dimorphic ornamentation. Focusing on the earliest stages of divergence, we tested preference for conspecific mates in males and females of seven closely related species pairs. Contrary to expectation, male preference for conspecific females was significantly greater than female preference for conspecific males. Males in four of the 14 species significantly preferred conspecific females; whereas, females in no species significantly preferred conspecific males. Relationships between the strength of preference for conspecifics and genetic distance revealed no difference in slope between males and females, but a significant difference in intercept, also suggesting that male preference evolves earlier than females’. Our results are consistent with other recent studies in darters and suggest that the coevolution of female preferences and male ornaments may not best explain the earliest stages of behavioral isolation in this lineage.